When you walk across the grassbound hills above Fox Creek, just northwest of the small town of Strong City, it's easy to pretend you're striding through the past. There is no sound or sight to remind you of the immediate century. But that reverie too is a way of failing to notice the grassland. The hard part here in the Flint Hills—and in any of the few remaining patches of native prairie—is learning to see the tallgrass ecosystem for itself. It is a study in the power of modesty. Learn it well enough and you begin to suspect that grasses are what hold this world together.

Some of the lowlands in the Flint Hills are planted to corn and milo, and the creek bottoms are full of oaks and an occasional white-limbed sycamore. Along the gravel roads you come across old limestone fences and Osage orange trees, or bois d'arc, planted by the settlers as hedge and windbreak. But on the uplands—and the Flint Hills are mostly upland, stretching from northern Kansas down into Oklahoma—the prairie still holds its own. The soils are too thin, too rock-strewn to make good farmland. Wherever you walk, you find drifts of limestone, like fallen grave markers, grass pushing through the holes that time has made in that soluble stone. The very toughness of this place—amply recorded by its early occupants—has helped preserve it.

That toughness is more than mirrored in the grasses themselves, especially in species like big bluestem—one of the dominant grasses in the Flint Hills. Big bluestem (Andropogon gerardii) persists from year to year, spreading by seed and rhizome, creating an underground web of coarse roots near the surface as well as fine root fibers that may reach eight feet (two meters) deep where the soil allows. What grows above ground—the tillers—is essentially disposable. The actual growing tip of the plant lies low to the earth in spring and is undisturbed when the tillers are cropped or singed. Like most of the other plants in the tallgrass ecosystem, big bluestem actually rejoices in grazing and fire, if they come at the right time of year.

The prairie is sometimes called a sea of grass—a metaphor that points to the endless green expanse and the wavelike motion of the grasses. But there is also a tide in these grasslands—a cycle of growth that sweeps chronologically across the Flint Hills. High tide comes early in the year. The prairie swells into life in early spring, and if fire comes then, the plants respond with a redoubled burst of growth. The fires clear away last year's plant debris, letting in more light and warming the soil.

To the grazers on the hills—bison once and cattle now—the new green on blackened ground is a timely feast. In the cycle of warm-season grasses like big bluestem, this is the peak of their nutritional richness. They continue to grow all through the summer, but as the weeks pass they harden off until, in autumn, their leaves are somber, dried remnants of themselves, crimson, maroon, clattering in the wind.

In most of America, agriculture has meant replacing the incredible complexity of a natural ecosystem with the incredible simplicity of a single crop growing on bare ground. And almost everywhere, fire was the first thing banished, hunted down as relentlessly as if it were a sheep-killing wolf. But in the Flint Hills fire still thrives because the ranchers here depend on a natural ecosystem. Even in the mid-19th century, cattlemen understood that the richness of the Flint Hills grasses depended on a good spring burn—something they learned from the Native Americans they displaced. And so, early each April—even as the cattle that will graze down the prairie are shipping in—the hills go up in smoke.